Chemical oxygen generators are used for emergency systems, e.g. in aircraft, breathing apparatus for emergency services such as fire-fighters and mine rescue crews, in submarines, and other situations in which a compact emergency oxygen generator with a long shelf life is needed. They release oxygen via a chemical reaction, typical oxygen sources being inorganic, especially alkaline metal or alkaline earth metal, superoxides, chlorates, or perchlorates.
Chlorate candles, also known as oxygen candles, are examples of chemical oxygen generators that use chlorates or perchlorates as the oxygen source. Chlorate candles can produce oxygen at a fixed rate at about 6.5 man-hours of oxygen per kilogram of the mixture and have a long shelf life.
In addition to the oxygen-delivering compounds, additives with different properties are used in chemical oxygen generators, for example: fuels, catalysts, binders and moderators. Fuels support the oxygen-producing reaction and are typically metal powders, e.g. powders of iron, tin, manganese, cobalt, nickel, tungsten, titanium, magnesium, aluminium, niobium, zirconium, and/or mixtures thereof.
Transition metal oxides are typically used to catalyse the exothermic oxygen-producing reaction, particularly to reduce the temperature at which it takes place and thus reduce the heat released by the device in action. A commonly used catalyst is cobalt oxide, due to its ability to reduce the temperature of the decomposition reaction, e.g. of sodium chlorate to 290-400° C. Cobalt oxide is however, toxic and expensive, and thus it is necessary to use it in very low amounts. Although effective at low amounts (e.g. 0.1 wt. %), achieving uniform mixing at such low concentrations is difficult. Non-uniform mixing results in erratic performance, such as non-uniform oxygen production, which is clearly undesirable. Other commonly-used catalysts are manganese oxides, although these can produce high concentrations of chlorine in the oxygen produced. There thus exists a need to provide alternative catalysts for oxygen generation which overcome these problems.
Chemical oxygen generators are required to produce an enduring and continuous oxygen flow. For this reason, moderators are also used, i.e. to avoid unwanted side reaction products like chlorine during the decomposition process and/or to ensure a sufficient and substantially uniform oxygen flow. BaO2 is often used as a moderator, but this is toxic and thus disposal of scrap is expensive.
Chemical oxygen generators, i.e. chemical oxygen-generating devices, usually comprise moulds, i.e. containers or pellets which comprise the chemicals; these moulds, containers or pellets obviously need to remain structurally stable before and during use of the oxygen generator in order to avoid failure during the initial firing process and to avoid interruption of oxygen flow which could occur due to any mechanical changes of the generator body induced by environmental effects or the reaction's progress. Binders are therefore used in order to stabilise the body of the chemical oxygen generator, e.g. the chlorate candle body, and ensure that it remains safe during use. Typical binders are mica, glass powder, glass fibre, fibreglass, ceramic fibre, bentonite, kaolinite and mixtures thereof, although other inorganic binders can also be suitable. These add undesirable extra bulk to the compositions used to produce oxygen.
Decomposition of chlorates to release oxygen is exothermic, for example sodium chlorate decomposes at 500 to 600° C. Due to the high temperatures involved, chemical oxygen generators require thermal insulation to protect surrounding equipment. Such insulation adds further bulk to the oxygen generator, which is clearly undesirable as they may need to be stored for long periods, usually in locations (e.g. aircraft, submarines, fire engines) where space and weight capacity are limited. There is thus a need to reduce the size and/or weight of oxygen generators.
The present inventors have surprisingly found that certain oxide compounds are multi-functional, acting as catalyst, binder and fuel. These compounds therefore enable the production of lighter and more compact oxygen generators. Moreover, as the compounds are non-toxic, the above-mentioned problems with commonly used toxic catalysts are also avoided.
Thus, viewed from a first aspect, the present disclosure provides an oxygen-generating composition comprising an oxygen source and a mixed-metal oxide of formula: (Li,Fe,Mg)O. Preferably the mixed-metal oxide is in the form of nano-particles.
The degree of crystallinity of the mixed-metal oxides can be increased by thermal treatment, e.g. calcination. However, the best catalytic performance has been found for oxides of the present disclosure which are non-crystalline, e.g. semi-crystalline or amorphous. The oxides of the present disclosure are therefore preferably uncalcinated, i.e. they have not been thermally treated such that the degree of crystallinity is increased.
Preferably less than 90% (e.g. wt. %), especially less than 75%, particularly less than 50%, especially preferably less than 25% or less than 10% of the mixed-metal oxide material described herein is crystalline. Especially preferably the mixed-metal oxide of formula: (Li,Fe,Mg)O is substantially semi-crystalline or substantially amorphous.
Crystallinity can be measured using X-ray diffraction (XRD) or any other techniques known in the art, such as differential scanning calorimetry. Crystallinity involves both a short-range and long-range order of periodically occurring atoms inside the crystal lattice which results in characteristic well-defined X-ray powder patterns. Mixed-metal oxides of formula: (Li,Fe,Mg)O which have not been subjected to thermal treatment exhibit low degrees of crystallinity and can be referred to as amorphous or semi-crystalline. Such materials contain only short-range order of the atoms which results in undefined X-ray powder patterns which lack the significant reflexes found for more crystalline forms.
Preferably the mixed-metal oxide as herein described is in the form of a powder. Especially preferably the mixed-metal oxide is in the form of nano-particles, e.g. a nano-particulate powder.
The notation “(Li,Fe,Mg)O” is intended to denote a single chemical entity, rather than, for example, a mixture of lithium oxide(s), iron oxide(s) and magnesium oxide(s). The compositions of the present disclosure therefore comprise a mixed-metal lithium-iron-magnesium oxide, rather than a mixture of these metal oxides. The oxides of the present disclosure can therefore be viewed as Li- and Fe-doped MgO, i.e. MgO in which some magnesium cations are substituted by iron cations and lithium cations in the crystal lattice. Although the oxide could be doped FeO, doping of MgO is, however, most preferred. In the oxides of the present disclosure, iron typically has an oxidation state of +2 and/or +3.
The content of each element may be determined by standard techniques, e.g. atomic absorption spectroscopy or inductively coupled plasma atomic emission spectroscopy. Preferred examples of the mixed-metal oxides of the present disclosure are (Fe,Li,Mg)O, where the iron and lithium contents are independently 0.01 to 30 Atom % (at. %), especially 0.05 to 20 at. % (e.g. 0.1 to 10 or 0.1 to 1 at. %), especially preferably 0.1 to 0.5 at. %, e.g. around 0.45 at. %.
An alternative notation would be (Fex,Liy,Mg1-x-y)O, where x and y are independently preferably 0.0002 to 0.6; especially 0.001 to 0.4, especially preferably 0.002 to 0.01, e.g. around 0.009. Especially preferably x=y, i.e. Li and Fe are substantially equimolar.
Thermal treatment below <1050° C. does not change the relative proportions of the metals.
The inventors have developed a process for producing the mixed-metal oxides of the present disclosure in nano-particulate form. The use of (or method of using) such nano-particles in oxygen production, e.g. as binders, fuels and/or catalysts/moderators in oxygen production, forms a further novel aspect of this disclosure. Thus, in a preferred aspect, the mixed-metal oxides herein described are in the form of particles having a diameter (e.g. the average particle diameter) of less than or equal to 500 nm, preferably less than or equal to 300 nm, especially less than or equal to 200 nm. Smaller particles allow for better mixing in oxygen-generating compositions and thus more uniform oxygen production.
The mixed-metal oxides of the present disclosure may be characterised using standard techniques, such as surface area analysis, absorption spectroscopy, inductively coupled plasma atomic emission spectroscopy, X-ray diffraction, electron paramagnetic resonance nuclear magnetic resonance, scanning electron microscopy and/or transmission electron microscopy.
The specific surface area of the mixed-metal oxide materials of the present disclosure may be determined using standard techniques, e.g. a Surface Area Analyzer with surface areas calculable by the method of Brunauer, Emmett and Teller. Typical BET surface areas are 5 to 50 m2/g, e.g. 10 to 40 m2/g, especially 25 to 35 m2/g.
Characterisation of the oxides by X-ray diffraction (XRD) is possible using standard techniques, e.g. CuKα1 radiation, with a wavelength 0.154 nm.
EPR experiments may be performed in conventional continuous wave (cw) as well as in pulsed mode.
In some aspects, the mixed-metal oxide can form up to 70 wt. % of the oxygen-generating composition, i.e. the composition may comprise 0.1 to 70 wt. %, preferably 0.1 to 10 wt. %, particularly preferably 0.2 to 5 wt. %, e.g. 2 to 4 wt. %, especially around 3 wt. % mixed-metal oxide as herein described, where the amount of mixed-metal oxide is expressed as weight percent of the oxygen-generating composition as a whole (i.e. regarding the total weight of the oxygen source, mixed-metal oxide and any other components).
The oxygen source may be any compound suitable for producing breathable oxygen. Metal (especially alkali metal or alkaline earth metal) halogenates (especially chlorates, perchlorates or mixtures thereof), peroxides or superoxides are suitable, especially those of lithium, sodium or potassium, e.g. KO2. Preferably the oxygen source is, or comprises, one or more compounds selected from alkali metal chlorates, alkali metal perchlorates, alkaline earth metal chlorates, alkaline earth metal perchlorates and mixtures thereof, especially preferably alkali metal chlorates and/or alkali metal perchlorates. Particularly preferred oxygen sources are those comprising sodium or lithium, particularly sodium chlorate and lithium perchlorate, e.g. the oxygen source is preferably sodium chlorate and/or lithium perchlorate.
The oxygen-generating composition of the disclosure typically comprises 30 to 99.9 wt. % oxygen source, preferably 40 to 99 wt. %, especially preferably 70 to 99 wt. %, e.g. at least 80 wt. % or at least 95 wt. %, especially 90 to 99.9 wt. % where the amount of oxygen source is expressed as weight percent of the oxygen-generating composition as a whole (i.e. regarding the total weight of the oxygen source, mixed-metal oxide and any other components).
It is a particularly preferred aspect of all embodiments of this disclosure that the oxygen produced is breathable, e.g. without further treatment.
As discussed further in the Examples, it has been surprisingly found that the mixed-metal oxides of the present disclosure are multifunctional, acting as a catalyst as well as a fuel and binder. Thus, there is no need for additional fuel, binder, catalyst or moderator components in the oxygen-generating compositions of the present disclosure. This means that the compositions are simpler, quicker and cheaper to produce than conventional oxygen-generating compositions and that they are more compact and weigh less than those of the prior art (which require separate fuel, binder, catalyst and/or moderator components). This results in lighter and more compact oxygen-generators, e.g. chlorate candles. Yet further space and cost savings are enabled by the reduction in thermal insulation required for oxygen generators using the compositions of the present disclosure—less insulation being required due to a reduction in temperature of the oxygen-producing reaction due to the catalytic properties of the mixed-metal oxides disclosed herein. Moreover, the mixed-metal oxides of the present disclosure are non-toxic and enable more uniform oxygen production due to the more efficient mixing possible due to their nano-particulate nature.
For example, as shown in FIG. 2 and Example 2, uncalcinated (Li,Fe,Mg)O reduces the decomposition temperature of sodium chlorate to 340-400° C., i.e. the decomposition process finishes at a lower temperature than with cobalt oxide as a catalyst. The mixed-metal oxide of the present disclosure therefore performs better than cobalt oxide, which is commonly considered to be the best catalyst for this reaction. Furthermore, the mixed-metal oxide of the present disclosure also acts as a binder and fuel, thus removing the need for further components to perform these functions and is non-toxic.
Due to the multi-functional nature of the oxides herein described, the oxygen-generating compositions do not require the presence of separate fuels, catalysts, moderators or binders. Preferably the compositions as herein described therefore consist of, or consist essentially of the (one or more, preferably one) oxygen source and (one or more, preferably one of the) mixed-metal oxide described herein. However, although the compositions of the present disclosure do not require the presence of further components, one or more additional components may be present. Thus, one aspect of the present disclosure relates to the compositions as herein described additionally comprising one or more additives, e.g. fuels, catalysts, moderators and/or binders.
Thus, the compositions as herein described may further comprise one or more fuels. Metals or non-metals such as silicon, boron and/or carbon may be used. Preferably the fuel is in powder form, especially a metal powder, e.g. a powder of, or comprising, iron, tin, manganese, cobalt, nickel, tungsten, titanium, magnesium, aluminium, niobium or zirconium, and/or mixtures thereof. The compositions may optionally comprise 0 to 5 (e.g. 0.1 to 5) wt. % of such fuel (expressed as the weight percentage of the total weight of additional fuels as part of the composition as a whole), preferably 0 to 1 (e.g. 0.1 to 1) wt. %, especially 0 to 0.5 (e.g. 0.1 to 0.5) wt. %.
The compositions as herein described may further comprise one or more catalysts, e.g. a transition metal oxide, preferably selected from manganese oxides (e.g. MnO, Mn2O3), iron oxides (e.g. FeO and/or Fe2O3) cobalt oxide, copper oxide, nickel oxide and mixtures thereof. The compositions may optionally comprise 0 to 5 (e.g. 0.1 to 5) wt. % of such catalyst (expressed as the weight percentage of the total weight of additional catalysts as part of the composition as a whole), preferably 0 to 1 (e.g. 0.1 to 1) wt. %, especially 0 to 0.5 (e.g. 0.1 to 0.5) wt. %.
The compositions as herein described may further comprise one or more moderators, e.g. chlorine removers and/or reaction rate modifiers (e.g. inhibitors). These are preferably selected from the oxides, peroxides and hydroxides of alkali and alkaline earth metals, preferably barium peroxide. These compounds serve for binding chlorine and carbon dioxide, which are sometimes produced in trace amounts, but should not be present in breathable oxygen. They can also moderate the production of oxygen, ensuring a uniform supply. The compositions may optionally comprise 0 to 5 (e.g. 0.1 to 5) wt. % of such moderators (expressed as the weight percentage of the total weight of additional moderators as part of the composition as a whole), preferably 0 to 1 (e.g. 0.1 to 1) wt. %, especially 0 to 0.5 (e.g. 0.1 to 0.5) wt. %.
The compositions as herein described may further comprise one or more binders, preferably selected from inorganic binders such as mica, glass powder, glass fibre, fibreglass, ceramic fibre, steel wool, bentonite, kaolinite and mixtures thereof, for example, although other inorganic binders can also be suitable.
The oxygen-generating compositions as herein described may be made by combining the oxygen source with the mixed-metal oxide (and any other optional components). The disclosure therefore also provides a method for preparing an oxygen-generating composition as herein described, said method comprising combining the oxygen source with the mixed-metal oxide. Optionally the oxygen source is mixed with, e.g. 1-5 wt. %, water prior to being combined with the mixed-metal oxide. If any of the additional components mentioned herein are desired to be present in the composition, these would typically be mixed together with the mixed-metal oxide, preferably prior to combination with the oxygen source. The components may be combined by any suitable method, e.g. mixing. After the components have been combined, the resulting oxygen-generating composition may be dried and stored for future use, or placed in a mould to form part of an oxygen generator.
The mixed-metal oxide of the present disclosure may be prepared by any known route. Suitable routes for the manufacture of oxides of this type are found in Top. Catal. (2011) 54; 1266-1285. Most preferably, the mixed-metal oxide is prepared by co-precipitation, e.g. by adding aqueous solutions of salts such as Mg(NO3)2.6H2O and Fe(NO3)3.9H2O to a solution, preferably an ammonia solution (preferably at a pH of 11 or above) to precipitate a semi-crystalline powder, which is then further mixed with LiOH.H2O.
Other metal salts, e.g. chlorides or phosphates may be used for the precipitation step, but nitrates are especially preferred as they are completely removable by thermal treatment. Similarly, ammonia is the preferred medium for precipitation. If chlorides are used, the precipitate may need to be extensively washed to avoid the presence of chlorine.
In order to produce nano-particles, the resulting material may be quick-frozen using liquid N2, followed by freeze-drying, e.g. for over 12 hours, especially over 72 hours. The combination of precipitation, freezing and freeze-drying produces homogenous, high purity nano-particles.
Decomposition of the oxygen source of the compositions of the present disclosure results in oxygen production. Thus, viewed from a further aspect, the present disclosure provides the use of a mixed-metal oxide as herein described in a method for generating oxygen. The present disclosure also provides use of the mixed-metal oxides as herein described as multi-functional components in oxygen-generating compositions and oxygen generators, i.e. a single component with the functions of catalyst, binder and fuel. A further aspect of the present disclosure relates to a method for generating oxygen, said method comprising decomposing an oxygen source as herein described in the presence of a mixed-metal oxide as herein described.
The compositions of the present disclosure have utility in chemical oxygen generators, also referred to as “chemical oxygen generating devices”, “chemical oxygen systems”, “chemical oxygen generating systems”, “oxygen-generators” etc.
Thus, viewed from a further aspect, the present disclosure provides a chemical oxygen generator comprising an oxygen-generating composition as herein described. Preferably, said generator comprises a container for containing the oxygen-generating composition and a primer for starting decomposition of the oxygen-generating composition.
Typical chemical oxygen generators/devices according to the present disclosure are fixed chemical oxygen generators and portable chemical oxygen generators. Especially preferably the chemical oxygen generator is, or comprises, a chlorate candle.
Fixed chemical oxygen generators are used in fixed systems, e.g. those commonly used in passenger-carrying aircraft. The system typically comprises boxes, each carrying an oxygen generator and one or more passenger masks. The generators are activated as the masks are presented. Thus, the present disclosure also provides the use of the compositions and generators as herein described in a fixed oxygen generation system, e.g., an aircraft oxygen generation system. The present disclosure also provides a method for producing oxygen, e.g. in an aircraft, comprising use of the compositions and generators as herein described.
Aircraft typically carry one or more oxygen generating systems selected from continuous flow systems, demand flow systems, diluter demand systems and pressure demand systems. The generators and compositions of the present disclosure may be used in any such system. Thus, a further aspect of the disclosure provides an oxygen generating system, preferably an aircraft oxygen generating system, comprising a chemical oxygen generator or oxygen generating compositions as herein described.
The present disclosure also provides a kit for making an oxygen-generating composition as herein described or a kit for making a chemical oxygen generator as herein described, said kit comprising an oxygen source as herein described and a mixed-metal oxide as herein described.
Chemical oxygen-generating devices such as chlorate candles typically have a generally cylindrical shape with a taper, with a recess at one end to hold an ignition pellet. A typical candle configuration is shown in FIG. 1 of SAE AIR1133. The ignition pellet may be ignited by firing a primer. The heat from the ignition pellet then ignites the reaction of the candle body and generates oxygen. The oxygen-generating devices of the present disclosure therefore preferably further comprise an ignition pellet and/or primer. Suitable pellets and primers are known in the art.
The oxygen-generator, e.g. candle, may comprise several layers of different compositions and thus different reaction rates. Multiple layers can be used to help match the oxygen generation requirements. Different applications have different oxygen generation requirements. The interface shapes and relative sizes and reactivities of the layers can be modified, depending upon the requirements of the specific applications of the oxygen-generating device.
The oxygen-generating devices of the present disclosure may therefore comprise compositions in addition to the oxygen-generating composition as herein described. Alternatively, the devices could comprise a plurality of layers comprising the same or different compositions according to the present disclosure, e.g. layers comprising oxygen-generating compositions as herein described which differ from one another in one or more aspects such as: the identity/amount of the oxygen source; the identity (e.g. specific proportion of metals in the oxide and/or degree of crystallinity)/amount of mixed-metal oxide present; and/or the amount and/or identity of any optional components as herein described.
Formation of devices according to the present disclosure may be achieved by preparing the mixed-metal oxide by any of the methods referred to herein and separately mixing the oxygen source with approximately 1 to 5 wt. % water (the water being used as a lubricant to facilitate the formation of the oxygen-generating cores or candles). The mixed-metal oxide is then mixed with the wet oxygen source. The oxygen-generating device, e.g. oxygen-generating candle, may be formed by compaction of the damp mixture in a mould, which is then dried, e.g. at about 120° C., to remove the water that was added during the mixing process.
It will be appreciated that the disclosed uses and methods may take advantage of any of the materials described above in relation to the compositions and products and vice versa.
All references herein to “comprising” should be understood to encompass “including” and “containing” as well as “consisting of” and “consisting essentially of”.
As shown in FIG. 1, mixed-metal oxides according to the present disclosure can be formed by precipitation. An aqueous solution of Mg(NO3)2.6H2O is prepared by dissolving Mg(NO3)2.6H2O in distilled H2O. A solution of Fe(NO3)3.9H2O is prepared in the same way. The nitrate solutions are concurrently added drop-wise to a stirred ammonia solution while keeping the pH value above 11. The gelatinous precipitates can be separated by centrifugation and rinsed with distilled H2O in a “cleansing” step. In order to incorporate Li into the mixed-metal oxide, the resulting precipitate is mixed with an aqueous LiOH solution (LiOH.xH2O) with appropriate Li concentrations (chosen to influence the amount of Li in the eventual oxide) in a tubular mixer and homogenised. In order to produce nano-powder, the suspension can be quick-frozen using liquid nitrogen. Afterwards, it may be freeze-dried over at least 12 hours using a freeze-dryer. The combination the above mentioned precipitation, quick-freezing and freeze-drying steps produces nano-particles. Optionally a further crushing step may be used, although the method allows production of nano-particles without any crushing step.
It will be understood that the description above relates to a non-limiting example and that various changes and modifications may be made from the arrangement shown without departing from the scope of this disclosure, which is set forth in the accompanying claims.
The disclosure will now be further described by way of the following non-limiting Examples: